Some guidelines recommend against any alcohol consumption during pregnancy, while others suggest that moderate drinking might be safe. And previous studies have yielded conflicting findings about the effects that moderate maternal drinking can have on a child's IQ.

This new study included more than 4,000 women in the United Kingdom who provided information about their alcohol intake during pregnancy. Their children's IQ was tested when they were 8 years old.

The researchers found that four variations in alcohol-metabolizing genes in the children were strongly related to lower IQ. On average, a child's IQ was almost two points lower for each of the four genetic variants they had.

This effect was seen only among children of women who were moderate drinkers (one to six drinks per week) during pregnancy. It was not seen in children whose mothers did not drink during pregnancy.

This suggests that it was exposure to alcohol in the womb that led to the differences in the children's IQ, the University of Bristol and University of Oxford researchers said.

The study did not include women who were heavy drinkers during pregnancy.

The findings were published Nov. 14 in the journal PLoS One.

"Our results suggest that even at levels of alcohol consumption which are normally considered to be harmless, we can detect differences in childhood IQ, which are dependent on the ability of the fetus to clear this alcohol. This is evidence that even at these moderate levels, alcohol is influencing fetal brain development," study main author Dr. Sarah Lewis, at the University of Bristol, said in a university news release.

Study leader Dr. Ron Gray, at the University of Oxford, added, "This is a complex study but the message is simple: Even moderate amounts of alcohol during pregnancy can have an effect on future child intelligence. So women have good reason to choose to avoid alcohol when pregnant."

While the study found an association between a mother's drinking during pregnancy and a child's IQ, it did not prove a cause-and-effect relationship.